Governments must do more to protect children from sophisticated online methods used to market unhealthy foods to them.

By using digital analytics and geo-location data, companies were defining their audience with great precision, enabling them to target the most vulnerable groups of children, the UN agency warned.

And it urged governments to set a minimum legal age of 16 for advertising foods high in fat, salt or sugar.

Current regulations about advertising such foods often only apply to non-digital media, or only related to young children and not adolescents, it said.

"In digital media, an extensive, highly-complex system of advertising delivery has evolved, through which marketers can access much more specific audiences than in the broadcast era," the report said.

"For fast-food brands, geo-location data from mobile devices enable marketers to deliver ads and special offers in real time when users are in the area."

The aim of the industry's digital marketing was to "engage children in emotional, entertaining experiences and to encourage them to share these experiences with their friends," it said.

In the United States, the Children's Online Privacy Protection Act (COPPA) limits how much data can be collected from young children, and bans "behavioral advertising" directed at children under 13 without parental notice and consent.

But the agency said the law appeared to be largely ineffective since many parents gave their children permission to play games or join certain sites without realizing the implications of allowing their children's data to be collected.

Social media platforms and marketers have themselves reported that social media marketing amplifies the effects of broadcast marketing.

Establishing a minimum legal age for such advertising must be accompanied by "effective oversight and enforcement by regulatory agencies," it said.

"Children's participation in digital media should not be predicated on receiving digital (high fat, sugar or salt food) advertising, nor should it be predicated on 'devolving' consent to parents," it said.